Inferring threshold voltage distributions associated with memory cells via interpolation

ABSTRACT

An apparatuses and methods for inferring threshold voltage distributions associated with memory cells via interpolation. A determining soft data for a group of memory cells each programmed to one of a number of data states, wherein the soft data comprises a number of different soft data values, determining a quantity of memory cells associated with each of the different soft data values, and inferring at least a portion of a threshold voltage distribution associated with the group of memory cells via an interpolation process using the determined quantities of memory cells associated with each of the different soft data values.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor memory andmethods, and more particularly, to inferring threshold voltagedistributions associated with memory cells via interpolation.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits and/or external removable devices in computers orother electronic devices. There are many different types of memoryincluding volatile and non-volatile memory. Volatile memory can requirepower to maintain its information and can include random-access memory(RAM), dynamic random access memory (DRAM), and synchronous dynamicrandom access memory (SDRAM), among others. Non-volatile memory canprovide persistent information by retaining stored information when notpowered and can include NAND flash memory, NOR flash memory, phasechange random access memory (PCRAM), resistive random access memory(RRAM), and magnetic random access memory (MRAM), among others.

Memory devices can be combined together to form a solid state drive(SSD). An SSD can include non-volatile memory (e.g., NAND flash memoryand/or NOR flash memory), and/or can include volatile memory (e.g., DRAMand/or SRAM), among various other types of non-volatile and volatilememory. Flash memory devices can include memory cells storinginformation in a charge storage structure such as a floating gate, forinstance, and may be utilized as non-volatile memory for a wide range ofelectronic applications. Flash memory devices typically use aone-transistor memory cell that allows for high memory densities, highreliability, and low power consumption.

Memory cells in an array architecture can be programmed to a desiredstate. For instance, electric charge can be placed on or removed fromthe charge storage structure (e.g., floating gate) of a memory cell toprogram the cell to a particular data state. The stored charge on thecharge storage structure of the memory cell can indicate a thresholdvoltage (Vt) of the cell.

For example, a single level cell (SLC) can be programmed to a targetedone of two different data states, which can be represented by the binaryunits 1 or 0. Some flash memory cells can be programmed to a targetedone of more than two data states (e.g., 1111, 0111, 0011, 1011, 1001,0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, and 1110).Such cells may be referred to as multi state memory cells, multiunitcells, or multilevel cells (MLCs). MLCs can provide higher densitymemories without increasing the number of memory cells since each cellcan represent more than one digit (e.g., more than one bit).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a portion of a non-volatilememory array in accordance with a number of embodiments of the presentdisclosure.

FIG. 2 illustrates a diagram of a number of threshold voltagedistributions, sensing voltages, and data assignments in accordance witha number of embodiments of the present disclosure.

FIG. 3 illustrates a block diagram of an apparatus in the form of amemory device in accordance with a number of embodiments of the presentdisclosure.

FIGS. 4A-4B illustrate a diagram of a number of threshold voltagedistributions, sensing voltages, data assignments, and histograms inaccordance with a number of embodiments of the present disclosure.

FIGS. 5A-5C illustrate a diagram of a number of threshold voltagedistributions, sensing voltages, data assignments, and histograms inaccordance with a number of embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure includes apparatuses and methods for inferringthreshold voltage distributions associated with memory cells viainterpolation. A number of embodiments include determining soft data fora group of memory cells each programmed to one of a number of datastates, wherein the soft data comprises a number of different soft datavalues, determining a quantity of memory cells associated with each ofthe different soft data values, and inferring at least a portion of athreshold voltage distribution associated with the group of memory cellsvia an interpolation process using the determined quantities of memorycells associated with each of the different soft data values.

Hard data can refer to a binary data value stored in one or more memorycells and provided to a host responsive to a read operation, forexample. In various instances, soft data can also be determined inassociation with a read operation. For example, during a read operation,a ramped sensing signal can be applied to a selected memory cell, and apage buffer can latch a digital value corresponding to the amplitude ofthe sensing signal when the selected memory cell conducts (or stopsconducting, in the case of a falling slope read operation). The mostsignificant bit(s) of the digital value can correspond to the hard data(e.g., the data state of the selected cell), and the least significantbit(s) of the digital value can correspond to the associated soft data.Soft data can, for example, indicate the quality and/or confidence ofthe hard data (e.g., information regarding the probability of the cellstoring the read hard data or of the cell storing different data).Accordingly, soft data can provide benefits such as increased accuracyand/or reliability (e.g., decreased error rate), and/or increased memorylife, among other benefits.

For example, in a number of embodiments of the present disclosure, softdata associated with a number of memory cells can be used to infer athreshold voltage (Vt) distribution (e.g., a Vt distribution curve)associated with the number of memory cells via an interpolation process,as will be further described herein. Using interpolation to infer a Vtdistribution can provide an increased Vt distribution resolution ascompared to previous approaches (e.g., a 50 millivolt Vt distributionresolution as compared to a 200 millivolt Vt distribution resolution).Providing an increased (e.g., finer) Vt distribution resolution canprovide benefits such as enabling further signal processing (e.g.,valley detection), among other benefits.

In the following detailed description of the present disclosure,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration how a number of embodimentsof the disclosure may be practiced. These embodiments are described insufficient detail to enable those of ordinary skill in the art topractice the embodiments of this disclosure, and it is to be understoodthat other embodiments may be utilized and that process, electrical,and/or structural changes may be made without departing from the scopeof the present disclosure.

As used herein, “a number of” something can refer to one or more suchthings. For example, a number of memory cells can refer to one or morememory cells. Additionally, the designators “M” and “N” as used herein,particularly with respect to reference numerals in the drawings,indicates that a number of the particular feature so designated can beincluded with a number of embodiments of the present disclosure.

The figures herein follow a numbering convention in which the firstdigit or digits correspond to the drawing figure number and theremaining digits identify an element or component in the drawing.Similar elements or components between different figures may beidentified by the use of similar digits. For example, 100 may referenceelement “00” in FIG. 1, and a similar element may be referenced as 300in FIG. 3. As will be appreciated, elements shown in the variousembodiments herein can be added, exchanged, and/or eliminated so as toprovide a number of additional embodiments of the present disclosure. Inaddition, as will be appreciated, the proportion and the relative scaleof the elements provided in the figures are intended to illustrate theembodiments of the present disclosure, and should not be taken in alimiting sense.

FIG. 1 illustrates a schematic diagram of a portion of a non-volatilememory array 100 in accordance with a number of embodiments of thepresent disclosure. The embodiment of FIG. 1 illustrates a NANDarchitecture non-volatile memory array. However, embodiments describedherein are not limited to this example. As shown in FIG. 1, memory array100 includes access lines (e.g., word lines 105-1, . . . , 105-N) andintersecting data lines (e.g., local bit lines) 107-1, 107-2, 107-3, . .. , 107-M. For ease of addressing in the digital environment, the numberof word lines 105-1, . . . , 105-N and the number of local bit lines107-1, 107-2, 107-3, . . . , 107-M can be some power of two (e.g., 256word lines by 4,096 bit lines).

Memory array 100 includes NAND strings 109-1, 109-2, 109-3, . . . ,109-M. Each NAND string includes non-volatile memory cells 111-1, . . ., 111-N, each communicatively coupled to a respective word line 105-1, .. . , 105-N. Each NAND string (and its constituent memory cells) is alsoassociated with a local bit line 107-1, 107-2, 107-3, . . . , 107-M. Thenon-volatile memory cells 111-1, . . . , 111-N of each NAND string109-1, 109-2, 109-3, . . . , 109-M are connected in series source todrain between a source select gate (SGS) (e.g., a field-effecttransistor (FET)) 113, and a drain select gate (SGD) (e.g., FET) 119.Each source select gate 113 is configured to selectively couple arespective NAND string to a common source 123 responsive to a signal onsource select line 117, while each drain select gate 119 is configuredto selectively couple a respective NAND string to a respective bit lineresponsive to a signal on drain select line 115.

As shown in the embodiment illustrated in FIG. 1, a source of sourceselect gate 113 is connected to a common source line 123. The drain ofsource select gate 113 is connected to the source of the memory cell111-1 of the corresponding NAND string 109-1. The drain of drain selectgate 119 is connected to bit line 107-1 of the corresponding NAND string109-1 at drain contact 121-1. The source of drain select gate 119 isconnected to the drain of the last memory cell 111-N (e.g., afloating-gate transistor) of the corresponding NAND string 109-1.

In a number of embodiments, construction of non-volatile memory cells111-1, . . . , 111-N includes a source, a drain, a charge storagestructure such as a floating gate, and a control gate. Non-volatilememory cells 111-1, . . . , 111-N have their control gates coupled to aword line, 105-1, . . . , 105-N respectively. A “column” of thenon-volatile memory cells, 111-1, . . . , 111-N, make up the NANDstrings 109-1, 109-2, 109-3, . . . , 109-M, and are coupled to a givenlocal bit line 107-1, 107-2, 107-3, . . . , 107-M, respectively. A “row”of the non-volatile memory cells are those memory cells commonly coupledto a given word line 105-1, . . . , 105-N. The use of the terms “column”and “row” is not meant to imply a particular linear (e.g., verticaland/or horizontal) orientation of the non-volatile memory cells. A NORarray architecture would be similarly laid out, except that the stringof memory cells would be coupled in parallel between the select gates.

As one of ordinary skill in the art will appreciate, subsets of cellscoupled to a selected word line (e.g., 105-1, . . . , 105-N) can beprogrammed and/or sensed (e.g., read) together as a page of memorycells. A programming operation (e.g., a write operation) can includeapplying a number of program pulses (e.g., 16V-20V) to a selected wordline in order to increase the threshold voltage (Vt) of selected cellscoupled to that selected access line to a desired program voltage levelcorresponding to a target (e.g., desired) data state.

A sense operation, such as a read or program verify operation, caninclude sensing a voltage and/or current change of a bit line coupled toa selected cell in order to determine the data state of the selectedcell, as well as the soft data associated therewith. The sensingoperation can involve providing a voltage to (e.g., biasing) a bit line(e.g., bit line 107-1) associated with a selected memory cell above avoltage (e.g., bias voltage) provided to a source line (e.g., sourceline 123) associated with the selected memory cell. A sensing operationcould alternatively include precharging the bit line followed withdischarge when a selected cell begins to conduct, and sensing thedischarge.

Sensing the state of a selected cell can include providing a number oframped sensing signals (e.g., read voltages) to a selected word linewhile providing a number of pass signals (e.g., read pass voltages) tothe word lines coupled to the unselected cells of the string sufficientto place the unselected cells in a conducting state independent of theVt of the unselected cells. The bit line corresponding to the selectedcell being read and/or verified can be sensed to determine whether ornot the selected cell conducts in response to the particular sensingvoltage applied to the selected word line. For example, the data stateand/or associated soft data of a selected cell can be determined by theword line voltage at which the bit line current reaches a particularreference current associated with a particular state.

As one of ordinary skill in the art will appreciate, in a sensingoperation performed on a selected memory cell in a NAND string, theunselected memory cells of the string are biased so as to be in aconducting state. In such a sensing operation, the data state and/orassociated soft data of the selected cell can be determined based on thecurrent and/or voltage sensed on the bit line corresponding to thestring. For instance, the data state and/or associated soft data of theselected cell can be determined based on whether the bit line currentchanges by a particular amount or reaches a particular level in a giventime period.

When the selected cell is in a conductive state, current flows betweenthe source line contact at one end of the string and a bit line contactat the other end of the string. As such, the current associated withsensing the selected cell is carried through each of the other cells inthe string, the diffused regions between cell stacks, and the selecttransistors.

FIG. 2 illustrates a diagram 201 of a number of threshold voltage (Vt)distributions, sensing voltages, and data (e.g., hard and soft data)assignments in accordance with a number of embodiments of the presentdisclosure. The example shown in FIG. 2 can represent, for example,memory cells 111-1, . . . , 111-N previously described in connectionwith FIG. 1. For instance, the example shown in FIG. 2 can represent afirst (e.g., lower) page of two-bit (e.g., four-state) memory cells. Aswill be appreciated by one of ordinary skill in the art, an analogousrepresentation of the second (e.g., upper) page of the two-bit memorycells (not shown in FIG. 2 for simplicity) can include the Vtdistributions, sensing voltages, and data assignments of the first page,as well as Vt distributions, sensing voltages, and data assignments thatmirror those of the first page. However, embodiments of the presentdisclosure are not limited to this example of two-bit memory cells, aswill be appreciated by one of ordinary skill in the art.

As shown in FIG. 2, Vt distributions 225-1 and 225-2 represent twotarget data states (e.g., L1 and L2, respectively) to which the memorycells can be programmed. Target state L1 can correspond to data 1, andtarget state L2 can correspond to data 0. That is, the data state ofeach of the memory cells can correspond to one of a number of (e.g.,two) data values (e.g., hard data). For example, in the embodimentillustrated in FIG. 2, the first data state (e.g., L2) to which thememory cells can be programmed corresponds to data 0 (e.g., a hard datavalue of 0 can be output responsive to a read request), and the seconddata state (e.g., L1) to which the memory cells can be programmedcorresponds to data 1 (e.g., a hard data value of 1 can be outputresponsive to a read request). Embodiments of the present disclosure,however, are not limited to these particular data assignments.

Vt distributions 225-1 and 225-2 can represent a number of memory cellsthat are programmed to the corresponding target states, with the heightof a Vt distribution curve indicating a number of cells programmed to aparticular voltage within the Vt distribution (e.g., on average). Thewidth of the Vt distribution curve indicates the range of voltages thatrepresent a particular target state (e.g., the width of the Vtdistribution curve 225-2 for L2 represents the range of voltages thatcorrespond to hard data 0).

During a sense (e.g., read) operation, a sensing (e.g., read) voltagelocated between Vt distributions 225-1 and 225-2 can be used todistinguish between states L1 and L2. In a read operation performed on aselected memory cell in a NAND string, the unselected memory cells ofthe string can be biased with a pass voltage so as to be in a conductingstate. When all cells in a string are in a conductive state, current canflow between the source line contact at one end of the string and adrain line contact at the other end of the string. As such, the datastate of the selected cell can be determined based on the current and/orvoltage sensed on a bit line corresponding to a particular string whenthe selected cell begins to conduct (e.g., in response to the particularread voltage applied to the control gate of the cell (via a selectedword line)). For example, the data state of the selected cell can bedetermined based on whether the bit line current changes by a particularamount, or reaches a particular level within a given time period. Othertypes of read operations are also possible, as will be understood by oneof ordinary skill in the art.

Each data state (e.g., L1, and L2) of the memory cells can have softdata associated therewith. For instance, the Vt distribution (e.g.,225-1 or 225-2) associated with each data state can have soft data(e.g., bits) assigned thereto. As previously described herein, the softdata can correspond to the least significant bit(s) of a digital valuethat corresponds to the amplitude of a ramped sensing signal applied toa selected memory cell when the cell conducts (or stops conducting, inthe case of a falling slope read operation). In the example illustratedin FIG. 2, two bits are used to provide soft data (e.g., quality and/orconfidence information) associated with the data states.

Soft data associated with a data state of a memory cell can indicate alocation of the Vt associated with the memory cell within the Vtdistribution associated with the data state of the memory cell. Forexample, in the embodiment illustrated in FIG. 2, soft data 00associated with data state L2 indicates that the Vt of the memory cellis located at a voltage greater than read voltage R5 within Vtdistribution 225-2 (e.g., that the Vt of the memory cell is locatedtoward the middle of Vt distribution 225-2), and soft data 00 associatedwith data state L1 indicates that the Vt of the memory cell is locatedat a voltage less than read voltage R1 within Vt distribution 225-1(e.g., that the Vt of the memory cell is located toward the middle of Vtdistribution 225-1). Additionally, soft data 10 associated with datastate L2 indicates that the Vt of the memory cell is located at avoltage between read voltages R4 and R5 within Vt distribution 225-2,and soft data 10 associated with data state L1 indicates that the Vt ofthe memory cell is located at a voltage between read voltages R1 and R2(e.g., soft data 10 indicates that the Vt of the memory cell is locatedcloser toward the edge of the Vt distribution than soft data 00).Further, soft data 11 associated with data state L2 (e.g., hard data 0)indicates that the Vt of the memory cell is located at a voltage betweenread voltages R3 and R4. Additionally, soft data 11 associated with datastate L1 (e.g., hard data 1) indicates that the Vt of the memory cell islocated at a voltage between read voltages R2 and R3 (e.g., that thehard data may not match the target state to which the cell wasoriginally programmed).

Soft data associated with a data state of a memory cell can alsoindicate a probability of whether the Vt associated with the memory cellcorresponds to the data state of the memory cell. For example, in theembodiment illustrated in FIG. 2, soft data 00 associated with the datastate L2 indicates a strong probability that the Vt of the memory cellcorresponds to data state L2, soft data 10 associated with the datastate L2 indicates a moderate probability (e.g., a probability that isless than the strong probability) that the Vt of the memory cellcorresponds to data state L2, and soft data 11 associated with datastate L2 indicates a weak probability (e.g., a probability that is lessthan the moderate probability) that the Vt of the memory cellcorresponds to data state L2. Additionally, soft data 00 associated withthe data state L1 indicates a strong probability that the Vt of thememory cell corresponds to data state L1, soft data 10 associated withdata state L1 indicates a moderate probability that the Vt of the memorycell corresponds to data state L1, and soft data 11 associated with datastate L1 indicates a weak probability that the Vt of the memory cellcorresponds to data state L1.

Embodiments of the present disclosure are not limited to the readvoltages and/or soft data assignments shown in FIG. 2. For example, agreater number of read voltages and/or soft data assignments can be usedto indicate a more precise Vt location within a Vt distribution and/or amore precise probability of whether a Vt corresponds to a data state.However, for simplicity and so as not to obscure embodiments of thepresent disclosure, five read voltages and six soft data values (e.g.,six different soft data possibilities) representing six different harddata quality and/or confidence levels have been illustrated in FIG. 2.Additionally, although the read voltages are shown in FIG. 2 as beingspaced apart by the same voltage amount, embodiments of the presentdisclosure are not so limited (e.g., the read voltages can be spacedapart by different voltage amounts), as will be further describedherein.

The soft data associated with the data state of a memory cell can bedetermined, for example, by performing a number of sense (e.g., read)operations on the memory cell using a number of read voltages (e.g., R1through R5), as will be further described herein (e.g., in connectionwith FIGS. 4A-5C). The soft data can be used to infer the Vtdistribution (e.g., the Vt distribution curve) associated with thememory cell, as will be further described herein (e.g., in connectionwith FIGS. 4A-5C). In a number of embodiments, the read operation(s) canalso determine the data state of the memory cell. For instance, the datastate of the memory cell and the soft data associated with the datastate of the memory cell can both be determined by the same readoperation.

FIG. 3 illustrates a block diagram of an apparatus in the form of amemory device 330 in accordance with a number of embodiments of thepresent disclosure. As used herein, an “apparatus” can refer to, but isnot limited to, any of a variety of structures or combinations ofstructures, such as a circuit or circuitry, a die or dice, a module ormodules, a device or devices, or a system or systems, for example.

As shown in FIG. 3, memory device 330 includes a controller 332 coupledto a memory array 300. Memory array 300 can be, for example, memoryarray 100 previously described in connection with FIG. 1. Although onememory array is shown in FIG. 3, embodiments of the present disclosureare not so limited (e.g., memory device 330 can include more than onememory array coupled to controller 332).

Controller 332 can include, for example, control circuitry and/orfirmware. Controller 332 can be included on the same physical device(e.g., the same die) as memory array 300, or can be included on aseparate physical device that is communicatively coupled to the physicaldevice that includes memory array 300. In a number of embodiments,components of controller 332 can be spread across multiple physicaldevices (e.g., some components on the same die as the array, and somecomponents on a different die, module, or board).

In the embodiment illustrated in FIG. 3, controller 332 includes ahistogram builder 334, a normalization engine 336, an interpretationengine 338, and an interpolation engine 340. The functionality ofcontroller 332 will be further described herein (e.g., in connectionwith FIGS. 4A-5C). For example, controller 332 can use soft dataassociated with the memory cells of memory array 300 to infer the Vtdistribution (e.g., the Vt distribution curve) associated with thememory cells via an interpolation process, as will be further describedherein.

The embodiment illustrated in FIG. 3 can include additional circuitrythat is not illustrated so as not to obscure embodiments of the presentdisclosure. For example, memory device 330 can include address circuitryto latch address signals provided over I/O connectors through I/Ocircuitry. Address signals can be received and decoded by a row decoderand a column decoder, to access memory array 300. As an additionalexample, memory device 330 can include sense (e.g., read) circuitry.

FIGS. 4A and 5A illustrate diagrams 402 and 503, respectively, of anumber of threshold voltage (Vt) distributions, sensing voltages, data(e.g., soft data) assignments, and histograms in accordance with anumber of embodiments of the present disclosure. Vt distributions 425-1,425-2, 525-1, and 525-2 illustrated in FIGS. 4A and 5A, respectively,can be the actual (e.g., true) Vt distributions associated with thememory cells of memory arrays 100 and/or 300, and can be analogous to Vtdistributions 225-1 and 225-2, respectively, previously described inconnection with FIG. 2.

In a number of embodiments, controller 332 can determine (e.g., sense)soft data for a group of the memory cells (e.g., soft data associatedwith the data states of the group of memory cells). Each memory cell ofthe group can be programmed to one of a number of data states (e.g., L1or L2), as previously described herein. The soft data for each memorycell of the group can be one of a number of different soft data values(e.g., soft data possibilities), and can be analogous to the soft datapreviously described in connection with FIG. 2. For instance, asillustrated in FIGS. 4A and 5A, the different soft data values caninclude soft data 10 associated with data state L1, soft data 11associated with data state L1, soft data 11 associated with data stateL2, and soft data 10 associated with associated with data state L2, in amanner analogous to that previously described in connection with FIG. 2.That is, the number of different soft data values can be four. However,embodiments are not limited to a particular number of soft data valuesor soft data assignments. For example, in some embodiments, thedifferent soft data values can also include soft data 00 associated withdata state L1 and soft data 00 associated with data state L2, asillustrated in FIGS. 4A and 5A.

Controller 332 can determine the soft data for the group of memory cellsby performing a number of sense (e.g., read) operations on the memorycells using a number of sensing voltages. For example, controller 332can perform a number of separate (e.g., repeated) sense operations eachusing a different sensing voltage, or controller 332 can perform one(e.g., a single) sense operation using a ramped sensing signal.

The sensing voltages can be, for example, read voltages R1, R2, R3, R4,and R5 illustrated in FIGS. 4A and 5A. That is, the number of sensingvoltages can be five, and can be located around (e.g., near) the minimum(e.g., valley) of Vt distributions curves 425-1 and 425-2. However,embodiments are not limited to a particular number or location ofsensing voltages used to determine the soft data. For example, in anumber of embodiments, the sensing voltages can be located in arbitrarypositions along the Vt axis.

In a number of embodiments, the sensing voltages can be spaced apart bythe same voltage amount, as shown in the example illustrated in FIG. 4A.For example, the sensing voltages can be spaced apart by 200 millivolts(mV). However, embodiments are not limited to a particular voltagespacing amount.

In a number of embodiments, the sensing voltages can be spaced apart bydifferent voltage amounts, as shown in the example illustrated in FIG.5A. For instance, in the example illustrated in FIG. 5A, read voltagesR1 and R2 and read voltages R4 and R5 are spaced apart by a firstvoltage amount (e.g., 300 mV), and read voltages R2 and R3 and readvoltages R3 and R4 are spaced apart by a second voltage amount (e.g.,200 mV) that is different than the first voltage amount. However,embodiments are not limited to a particular different voltage amounts ora particular number of different voltage amounts. For example, in anumber of embodiments, each of the sensing voltages can be spaced apartby different voltage amounts.

Controller 332 (e.g., histogram builder 334) can then determine (e.g.,count) the quantity (e.g., amount) of memory cells associated with eachof the different soft data values. For example, histogram builder 334can create (e.g., build) a histogram for each of the different soft datavalues. For instance, in the examples illustrated in FIGS. 4A and 5A,histograms 445-1 and 555-1, respectively, correspond to soft data 10associated with data state L1, histograms 445-2 and 555-2, respectively,correspond to soft data 11 associated with data state histograms 445-3and 555-3, respectively, correspond to soft data 11 associated with datastate L2, and histograms 445-4 and 555-4, respectively, correspond tosoft data 10 associated with data state L2.

The height of each histogram (e.g., the quantity of memory cells withineach histogram) can correspond to the quantity of memory cellsassociated with the soft data value for that particular histogram. Forinstance, in the examples illustrated in FIGS. 4A and 5A, the height ofhistograms 445-1 and 555-1, respectively, correspond to the quantity ofmemory cells associated with soft data 10 associated with data state L1,the height of histograms 445-2 and 555-2, respectively, correspond tothe quantity of memory cells associated with soft data 11 associatedwith data state L1, the height of histograms 445-3 and 555-3,respectively, correspond to the quantity of memory cells associated withsoft data 11 associated with data state L2, and the height of histograms445-4 and 555-4, respectively, correspond to the quantity of memorycells associated with soft data 10 associated with data state L2.

The width of each histogram can correspond to the spacing between thesensing voltages used to determine the soft data for the group of memorycells. For instance, in the example illustrated in FIG. 4A, the widthsof histograms 445-1, 445-2, 445-3, and 445-4 are the same (e.g., 200mV). In the example illustrated in FIG. 5A, the widths of histograms555-1 and 555-4 are the same (e.g., 300 mV), and the widths ofhistograms 555-2 and 555-3 are the same (e.g., 200 mV). Further, becausethe widths of histograms 555-1 and 555-4 are greater than the widths ofhistograms 445-1 and 445-4 (e.g., because the spacing between readvoltages R1 and R2 and read voltages R4 and R5 is greater in the exampleillustrated in FIG. 5 than in the example illustrated in FIG. 4), theheights of histograms 555-1 and 555-4 are greater than the heights ofhistograms 445-1 and 445-4, respectively (e.g., more memory cells areassociated with soft data 10 in the example illustrated in FIG. 5 thanin the example illustrated in FIG. 4).

In embodiments in which the sensing voltages used to determine the softdata are spaced apart by the same voltage amount (e.g., the exampleillustrated in FIG. 4A), controller 332 (e.g., interpretation engine338) can assign a Vt value (e.g., single Vt values) to represent each ofthe spacings between the sensing voltages (e.g., each Vt interval alongthe x-axis of diagram 402). For instance, a first Vt value can be usedto represent the spacing between read voltages R1 and R2, a second valuecan be used to represent the spacing between read voltages R2 and R3, athird value can be used to represent the spacing between read voltagesR3 and R4, etc. That is, interpretation engine 338 can refine the x-axis(e.g., the Vt) of diagram 402.

Interpretation engine 338 can assign the Vt values by, for example,mapping the spacings between the sensing voltages. The mapping can be,for example, a base mapping (e.g., the assigned Vt value for a spacingcan correspond to the mid-point of the spacing), or a heuristic mapping(e.g., the assigned Vt value for a spacing can correspond to a weightedaverage of the spacing). Interpretation engine 338 may use the basemapping if, for example, it is expected that the quantity of memorycells within the histograms are distributed evenly within thehistograms, and interpretation engine 338 may use the heuristic mappingif, for example, it is expected that the quantity of memory cells withinthe histograms are distributed unevenly within the histograms. Theexpectation of the distribution of the quantity of memory cells withinthe histograms can be based on, for example, a previously determined Vtdistribution associated with the group of memory cells.

Controller 332 (e.g., interpolation engine 340) can then infer (e.g.,approximate) at least a portion of a Vt distribution (e.g., at least aportion of a Vt distribution curve) associated with the group of memorycells via an interpolation process using the determined quantities ofmemory cells associated with each of the different soft data values andthe Vt values assigned to represent each of the spacings between thesensing voltages. The interpolation process can include, for example,fitting the quantities of memory cells and the assigned Vt values to acurve. The fitted curve can form at least the portion of the Vtdistribution (e.g., the Vt distribution curve).

An example of a portion of the inferred Vt distribution is illustratedin FIG. 4B. In the example illustrated in FIG. 4B, the inferred Vtdistribution includes Vt distributions (e.g., Vt distribution curves)447-1 and 447-2 that correspond to data states L1 and L2, respectively.As shown in FIG. 4B, the inferred Vt distributions 447-1 and 447-2 mayclosely approximate the actual Vt distributions 425-1 and 425-2,respectively, but may not exactly match the actual Vt distributions.

In the example illustrated in FIG. 4B, the interpolation process is acubic spline interpolation process. However, embodiments of the presentdisclosure are not limited to a particular type of interpolationprocess. For example, the interpolation process could be a linearinterpolation process or a polynomial interpolation process. The type ofinterpolation process can depend on, for example, the number ofdetermined quantities of memory cells (e.g., the number of differentsoft data values and/or the number of histograms created by histogrambuilder 334). Additionally, the closeness of the approximation of theinferred Vt distribution to the actual Vt distribution can depend on thetype of interpolation process. For instance, a polynomial interpolationprocess may provide a closer approximation to the actual Vt distributionthan a linear interpolation process, and a cubic spline interpolationprocess may provide a closer approximation to the actual Vt distributionthan a polynomial interpolation process.

Controller 332 can then determine, based on the inferred Vt distribution(e.g., the inferred Vt distribution curve), a sensing (e.g., read)voltage used to determine a data state (e.g., the hard data) of thegroup of memory cells. For example, the sensing voltage used todetermine a data state of the group of memory cells can correspond tothe minimum (e.g., the valley) of the inferred Vt distribution curve.For instance, in the example illustrated in FIG. 4B, the sensing voltagewould be the voltage along the Vt axis that corresponds to the minimumpoint 448 of the inferred Vt distribution curves 447-1 and 447-2. Thatis, in the example illustrated in FIG. 4B, the sensing voltage would beread voltage R3.

In a number of embodiments, controller 332 can determine the width ofthe inferred Vt distribution. That is, controller 332 can determine therange of voltages that represent the target state(s) represented by theinferred Vt distribution.

In embodiments in which the sensing voltages used to determine the softdata for the group of memory cells are spaced apart by different voltageamounts (e.g., the example illustrated in FIG. 5A), controller 332(e.g., normalization engine 336) can normalize the determined quantitiesof memory cells associated with each of the different soft data values.Normalizing the determined quantities of memory cells associated witheach of the different soft data values can include, for example,equalizing the weight (e.g., the statistical weight) of the determinedquantities of memory cells associated with each of the different softdata values. For instance, in the example illustrated in FIG. 5A, thenormalization can include adjusting (e.g., transforming) the quantity ofmemory cells associated with soft data value 10 (e.g., the quantity ofmemory cells within the wider histograms 555-1 and 555-4) to the samescale and/or level as the quantity of memory cells associated with softdata value 00 (e.g., the quantity of memory cells within the narrowerhistograms 555-2 and 555-3) in order to compensate for the differentsensing voltage spacings. That is, normalization engine 336 can refinethe y-axis (e.g., the histogram heights) of diagram 503.

For example, normalization engine 336 can normalize the determinedquantities of memory cells associated with each of the different softdata values by adjusting the heights of a number of the histogramscreated by histogram builder 334 such that the weight of the quantitiesof memory cells associated with each of the different soft data valuesis equal. For instance, in the example illustrated in FIG. 5B, theheights of histograms 555-1 and 555-4 are decreased to form normalizedhistograms 556-1 and 556-4, respectively, while the heights ofhistograms 555-2 and 555-3 remain the same, forming normalizedhistograms 556-2 and 556-3, respectively.

The height of each histogram after the heights of the number of thehistograms are adjusted (e.g., the height of each normalized histogram)can correspond to the normalized quantities of memory cells associatedwith each of the different soft data values. For instance, in theexample illustrated in FIG. 5B, the height of normalized histogram 556-1corresponds to the normalized quantity of memory cells associated withsoft data 10 associated with data state L1, the height of normalizedhistogram 556-2 corresponds to the normalized quantity of memory cellsassociated with soft data 11 associated with data state L1, the heightof normalized histogram 556-3 corresponds to the normalized quantity ofmemory cells associated with soft data 11 associated with data state L2,and the height of normalized histogram 556-4 corresponds to thenormalized quantity of memory cells associated with soft data 10associated with data state L2.

Controller 332 (e.g., interpretation engine 338) can then assign a Vtvalue (e.g., single Vt values) to represent each of the spacings betweenthe sensing voltages (e.g., each Vt interval along the x-axis of diagram503). For instance, in the example illustrated in FIG. 5B, a first Vtvalue can be used to represent the spacing between read voltages R1 andR2, a second value can be used to represent the spacing between readvoltages R2 and R3, etc. Interpretation engine 338 can assign the Vtvalues by, for example, mapping the spacings between the sensingvoltages in a manner analogous to that previously described herein.

Controller 332 (e.g., interpolation engine 340) can then infer (e.g.,approximate) at least a portion of a Vt distribution (e.g., at least aportion of a Vt distribution curve) associated with the group of memorycells by performing an interpolation process using the normalizedquantities of memory cells associated with each of the different softdata values and the Vt values assigned to represent each of the spacingsbetween the sensing voltages. The interpolation can include, forexample, fitting the normalized quantities of memory cells and theassigned Vt values to a curve. The fitted curve can form at least theportion of the inferred Vt distribution (e.g., the inferred Vtdistribution curve).

An example of a portion of the inferred Vt distribution is illustratedin FIG. 5C. In the example illustrated in FIG. 5C, the inferred Vtdistribution includes Vt distributions (e.g., Vt distribution curves)557-1 and 557-2 that correspond to data states L1 and L2, respectively.As shown in FIG. 5C, the inferred Vt distributions 557-1 and 557-2 mayclosely approximate the actual Vt distributions 525-1 and 525-2,respectively, but may not exactly match the actual Vt distributions.

In the example illustrated in FIG. 5C, the interpolation process is acubic spline interpolation process. However, embodiments of the presentdisclosure are not limited to a particular type of interpolation process(e.g., the interpolation process can be a linear or a polynomialinterpolation process), as previously described herein.

Controller 332 can then determine, based on the inferred Vtdistribution, a sensing (e.g., read) voltage used to determine a datastate (e.g., the hard data) of the group of memory cells. For example,the sensing voltage used to determine a data state of the group ofmemory cells can correspond to the minimum (e.g., the valley) of theinferred Vt distribution curve. For instance, in the example illustratedin FIG. 5C, the sensing voltage would be the voltage along the Vt axisthat corresponds to the minimum point 558 of the inferred Vtdistribution curves 557-1 and 557-2. That is, in the example illustratedin FIG. 5C, the sensing voltage would be read voltage R3. In a number ofembodiments, controller 332 can determine the width of the inferred Vtdistribution, as previously described herein.

Using interpolation to infer Vt distributions as described herein canprovide an increased Vt distribution resolution. The increased Vtresolution can, for example, increase the ability to accurately definemaxima (e.g., peaks) and/or minima (e.g., valleys) associated with theVt distributions.

CONCLUSION

The present disclosure includes apparatuses and methods for inferringthreshold voltage distributions associated with memory cells viainterpolation. A number of embodiments include determining soft data fora group of memory cells each programmed to one of a number of datastates, wherein the soft data comprises a number of different soft datavalues, determining a quantity of memory cells associated with each ofthe different soft data values, and inferring at least a portion of athreshold voltage distribution associated with the group of memory cellsvia an interpolation process using the determined quantities of memorycells associated with each of the different soft data values.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of a number of embodiments of the presentdisclosure. It is to be understood that the above description has beenmade in an illustrative fashion, and not a restrictive one. Combinationof the above embodiments, and other embodiments not specificallydescribed herein will be apparent to those of ordinary skill in the artupon reviewing the above description. The scope of a number ofembodiments of the present disclosure includes other applications inwhich the above structures and methods are used. Therefore, the scope ofa number of embodiments of the present disclosure should be determinedwith reference to the appended claims, along with the full range ofequivalents to which such claims are entitled.

In the foregoing Detailed Description, some features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

What is claimed is:
 1. A method for operating memory, comprising:determining soft data for a group of memory cells each programmed to oneof a number of data states, wherein the soft data comprises a number ofdifferent soft data values; determining a quantity of memory cellsassociated with each of the different soft data values; and inferring atleast a portion of a threshold voltage distribution associated with thegroup of memory cells via an interpolation process using the determinedquantities of memory cells associated with each of the different softdata values.
 2. The method of claim 1, wherein the method includes:normalizing the determined quantities of memory cells associated witheach of the different soft data values; and inferring at least theportion of the threshold voltage distribution associated with the groupof memory cells via the interpolation process using the normalizedquantities of memory cells associated with each of the different softdata values.
 3. The method of claim 1, wherein the method includesdetermining a sensing voltage used to determine a data state of thegroup of memory cells based on the inferred portion of the thresholdvoltage distribution associated with the group of memory cells.
 4. Themethod of claim 1, wherein determining the soft data for the group ofmemory cells includes performing a number of sense operations on thegroup of memory cells using a number of sensing voltages, wherein eachof the number of sensing voltages are spaced apart by a same voltageamount.
 5. The method of claim 1, wherein determining the quantity ofmemory cells associated with each of the different soft data valuesincludes creating a histogram for each of the different soft datavalues, wherein a height of each histogram corresponds to the quantityof memory cells associated with the soft data value for that particularhistogram.
 6. The method of claim 5, wherein a width of each histogramcorresponds to a spacing between sensing voltages used to determine thesoft data for the group memory cells.
 7. The method of claim 1, whereinthe method includes: determining the soft data associated with the groupof memory cells using a number of sensing voltages, wherein each of thesensing voltages are spaced apart by a voltage amount; assigning athreshold voltage value to represent each of the spacings between thesensing voltages; and inferring at least the portion of the thresholdvoltage distribution associated with the group of memory cells via theinterpolation process using the threshold voltage values assigned torepresent each of the spacings between the sensing voltages.
 8. Themethod of claim 7, wherein assigning a threshold voltage value torepresent each of the spacings between the sensing voltages includesmapping the spacings between the sensing voltages.
 9. The method ofclaim 8, wherein the mapping of the spacings between the sensingvoltages includes a base mapping of the spacings between the sensingvoltages.
 10. The method of claim 8, wherein the mapping of the spacingsbetween the sensing voltages includes a heuristic mapping of thespacings between the sensing voltages.
 11. The method of claim 8,wherein the mapping of the spacings between the sensing voltages isbased on a previously determined threshold voltage distributionassociated with the group of memory cells.
 12. The method of claim 7,wherein the interpolation process using the threshold voltage valuesassigned to represent each of the spacings between the sensing voltagesincludes fitting the assigned threshold voltage values to a curve. 13.The method of claim 1, wherein the method includes: using a controllerto determine the soft data for the group of memory cells; using acontroller to determine the quantity of memory cells associated witheach of the different soft data values; and using a controller to inferthe at least a portion of the threshold voltage distribution associatedwith the group of memory cells via the interpolation process.
 14. Amethod for operating memory, comprising: determining, using a number ofsensing voltages spaced apart by a voltage amount, soft data for a groupof memory cells each programmed to one of a number of data states,wherein the soft data comprises a number of different soft data values;determining the quantity of memory cells associated with each of thedifferent soft data values; normalizing the determined quantities ofmemory cells associated with each of the different soft data values;assigning a threshold voltage value to represent each of the spacingsbetween the sensing voltages; and inferring at least a portion of athreshold voltage distribution associated with the group of memory cellsvia an interpolation process using the threshold voltage values assignedto represent each of the spacings between the sensing voltages and thenormalized quantities of memory cells associated with each of thedifferent soft data values.
 15. The method of claim 14, wherein a numberof the number of sensing voltages are spaced apart by different voltageamounts.
 16. The method of claim 14, wherein normalizing the determinedquantities of memory cells associated with each of the different softdata values includes equalizing a weight of the determined quantities ofmemory cells associated with each of the different soft data values. 17.The method of claim 14, wherein the number of different soft data valuesis four.
 18. The method of claim 14, wherein inferring at least theportion of the threshold voltage distribution associated with the groupof memory cells includes inferring a threshold voltage distributioncurve associated with the group of memory cells.
 19. The method of claim14, wherein the method includes determining a width of the portion ofthe threshold voltage distribution associated with the group of memorycells.
 20. The method of claim 14, wherein the method includes: using ahistogram builder to determine the quantity of memory cells associatedwith each of the different soft data values; using a normalizationengine to normalize the determined quantities of memory cells associatedwith each of the different soft data values; using an interpretationengine to assign the threshold voltage values to represent the spacingsbetween the sensing voltages; and using an interpolation engine to inferthe at least a portion of the threshold voltage distribution associatedwith the group of memory cells via the interpolation process.
 21. Anapparatus, comprising: an array of memory cells; and a controllercoupled to the array and configured to: determine soft data for a groupof the memory cells each programmed to one of a number of data states,wherein the soft data comprises a number of different soft data values;determine a quantity of memory cells associated with each of thedifferent soft data values; and perform an interpolation process usingthe determined quantities of memory cells associated with each of thedifferent soft data values to infer at least a portion of a thresholdvoltage distribution curve associated with the group of memory cells.22. The apparatus of claim 21, wherein the controller is configured todetermine the soft data for the group of the memory cells using a numberof sensing voltages, wherein each of the number of sensing voltages arespaced apart by a same voltage amount.
 23. The apparatus of claim 21,wherein the controller is configured to determine a sensing voltage usedto determine a data state of the group of the memory cells based on theinferred threshold voltage distribution curve associated with the groupof memory cells.
 24. The apparatus of claim 23, wherein the sensingvoltage used to determine a data state of the group of the memory cellscorresponds to a minimum of the inferred threshold voltage distributioncurve.
 25. An apparatus, comprising: an array of memory cells; acontroller coupled to the array and configured to: determine soft datafor a group of the memory cells each programmed to one of a number ofdata states, wherein the soft data comprises a number of different softdata values; and determine a quantity of memory cells associated witheach of the different soft data values; and an interpolation engineconfigured to infer at least a portion of a threshold voltagedistribution associated with the group of the memory cells via aninterpolation process using the determined quantities of memory cellsassociated with each of the different soft data values.
 26. Theapparatus of claim 25, wherein the controller is on a same semiconductordie as the array.
 27. The apparatus of claim 25, wherein the controlleris on a different semiconductor die than the array.
 28. The apparatus ofclaim 25, wherein the controller includes circuitry on a samesemiconductor die as the array and circuitry on a differentsemiconductor die than the array.
 29. The apparatus of claim 25, whereinthe controller includes the interpolation engine.
 30. The apparatus ofclaim 25, wherein the interpolation process is a linear interpolationprocess.
 31. The apparatus of claim 25, wherein the interpolationprocess is a polynomial interpolation process.
 32. The apparatus ofclaim 25, wherein the interpolation process is a cubic splineinterpolation process.
 33. An apparatus, comprising: an array of memorycells; and a controller coupled to the array and configured to:determine, using a number of sensing voltages spaced apart by a voltageamount, soft data for a group of the memory cells each programmed to oneof a number of data states, wherein the soft data comprises a number ofdifferent soft data values; determine a quantity of memory cellsassociated with each of the different soft data values; normalize thedetermined quantities of memory cells associated with each of thedifferent soft data values; assign a threshold voltage value torepresent each of the spacings between the sensing voltages; and performan interpolation process using the threshold voltage values assigned torepresent the spacings between the sensing voltages and the normalizedquantities of memory cells associated with each of the different softdata values to infer at least a portion of a threshold voltagedistribution curve associated with the group of the memory cells. 34.The apparatus of claim 33, wherein the controller includes: a histogrambuilder configured to determine the quantity of memory cells associatedwith each of the different soft data values; a normalization engineconfigured to normalize the determined quantities of memory cellsassociated with each of the different soft data values; aninterpretation engine configured to assign the threshold voltage valuesto represent the spacings between the sensing voltages; and aninterpolation engine configured to perform the interpolation processusing the threshold values assigned to represent the spacings betweenthe sensing voltages and the normalized quantities of memory cellsassociated with each of the different soft data values to infer at leastthe portion of the threshold voltage distribution curve associated withthe group of the memory cells.
 35. The apparatus of claim 33, whereinthe controller is configured to: determine the quantity of memory cellsassociated with each of the different soft data values by creating ahistogram for each of the different soft data values, wherein a heightof each histogram corresponds to the quantity of memory cells associatedwith the soft data value for that particular histogram; and normalizethe determined quantities of memory cells associated with each of thedifferent soft data values by adjusting the heights of a number of thehistograms such that a weight of the quantities of memory cellsassociated with each of the different soft data values is equal, whereinthe height of each histogram after adjusting the heights of the numberof the histograms corresponds to the normalized quantities of memorycells associated with each of the different soft data values.
 36. Theapparatus of claim 35, wherein adjusting the heights of the number ofthe histograms includes decreasing the heights of the number of thehistograms.
 37. The apparatus of claim 33, wherein a number of thenumber of sensing voltages are spaced apart by different voltageamounts.
 38. The apparatus of claim 37, wherein: a first and a second ofthe number sensing voltages are spaced apart by a first voltage amount;the second and a third of the number of sensing voltages are spacedapart by a second voltage amount that is different than the firstvoltage amount; the third and a fourth of the number of sensing voltagesare spaced apart by the second voltage amount; and the fourth and afifth of the number of sensing voltages are spaced apart by the firstvoltage amount.
 39. The apparatus of claim 33, wherein each of thenumber of sensing voltages are spaced apart by different voltageamounts.